Generally known electrophotographic photoreceptors are comprised of chalcogenide glass, amorphous silicon, and organic photoconductive materials. With attention to a wide selection of organic photoconductive materials, a number of organic electrophotographic photoreceptors have recently been proposed. In particular, many of so-called separate functional type organic photoreceptors composed of a charge generating layer and a charge transporting layer have excellent characteristics. The charge transporting layer of separate functional type organic photoreceptors has been formed by a coating method in which a coating composition containing a charge transporting material, e.g., hydrazone derivatives, and a binder resin, e.g., polycarbonate resins, dissolved in an appropriate solvent is coated on a substrate by dip coating, wire bar coating, spray coating, or a like coating technique followed by drying.
However, the wet process coating method is liable to induce incorporation of impurities into a coating film, and the solvent of the coating composition is apt to remain to adversely affect electrophotographic characteristics and image quality retention properties. Further, the coating method is attended by a difficulty in film thickness control, often failing to form a charge transporting layer of uniform thickness.
For the purpose of avoiding these disadvantages of the coating method, a deposition method has been suggested, in which a charge transporting layer containing polyimide as a binder resin and an eutectic crystal of tetracyanoquinone and tetrathiofulvalene is formed by deposition polymerization, as disclosed in JP-A-1-214867 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In using the deposition polymerization method, however, the binder resin has insufficient transparency for obtaining satisfactory sensitivity. Further, there arises a problem of smeared image in a high humidity environment due to attachment of corona discharge products or talc.
Other photoconductive substances widely known include inorganic substances, e.g., amorphous selenium, selenium alloys, cadmium sulfide, and zinc oxide; and organic substances, e.g., polyvinylcarbazole and derivatives thereof. The organic photoconductive substances are advantageous over inorganic ones in terms of transparency, film-forming properties, flexibility, and easy production. Lately, electrophotographic photoreceptors containing various photoconductive pigments in the photosensitive layer thereof have also been proposed as disclosed, e.g., in JP-A-57-176046, JP-A-57-176047, and JP-A-2-37356. Such pigment-containing photoreceptors are produced by coating a coating composition comprising a binder resin, e.g., polycarbonate resins and polyester resins, a photoconductive pigment, and an appropriate solvent on a conductive substrate by dip coating or a like coating technique and then removing the solvent by drying.
However, the electrophotographic photoreceptors using pigments so far proposed have insufficient photosensitivity and suffer from a reduction in charging properties or an increase in residual potential on long-term use and are therefore unsatisfactory for practical use.
According to the inventors' study, it has been elucidated that the above-mentioned disadvantages of the state-of-the-art electrophotographic photoreceptors arise from the following facts: (1) Because of existence of a binder resin having per se no photoconductivity in a photosensitive layer, movement of electric charges is inhibited. (2) Impurities originated in solvents or binder resins, particularly chlorides and metallic elements remain in a photosensitive layer, and these impurities act as trapping centers or undergo electrochemical reactions with a pigment or a conductive substrate on repeated use.